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<ul id="index">
  <li><a href="#NAME">NAME</a></li>
  <li><a href="#SYNOPSIS">SYNOPSIS</a></li>
  <li><a href="#DESCRIPTION">DESCRIPTION</a></li>
  <li><a href="#NOTES">NOTES</a></li>
  <li><a href="#RETURN-VALUES">RETURN VALUES</a></li>
  <li><a href="#EXAMPLES">EXAMPLES</a></li>
  <li><a href="#BUGS">BUGS</a></li>
  <li><a href="#HISTORY">HISTORY</a></li>
  <li><a href="#COPYRIGHT">COPYRIGHT</a></li>
</ul>

<h1 id="NAME">NAME</h1>

<p>BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx - message digest BIO filter</p>

<h1 id="SYNOPSIS">SYNOPSIS</h1>

<pre><code> <span class="comment">#include &lt;openssl/bio.h&gt;</span>
 <span class="comment">#include &lt;openssl/evp.h&gt;</span>
 
 <span class="variable">const</span> <span class="variable">BIO_METHOD</span> <span class="variable">*BIO_f_md</span><span class="operator">(</span><span class="variable">void</span><span class="operator">);</span>
 <span class="keyword">int</span> <span class="variable">BIO_set_md</span><span class="operator">(</span><span class="variable">BIO</span> <span class="variable">*b</span><span class="operator">,</span> <span class="variable">EVP_MD</span> <span class="variable">*md</span><span class="operator">);</span>
 <span class="keyword">int</span> <span class="variable">BIO_get_md</span><span class="operator">(</span><span class="variable">BIO</span> <span class="variable">*b</span><span class="operator">,</span> <span class="variable">EVP_MD</span> <span class="operator">**</span><span class="variable">mdp</span><span class="operator">);</span>
 <span class="keyword">int</span> <span class="variable">BIO_get_md_ctx</span><span class="operator">(</span><span class="variable">BIO</span> <span class="variable">*b</span><span class="operator">,</span> <span class="variable">EVP_MD_CTX</span> <span class="operator">**</span><span class="variable">mdcp</span><span class="operator">);</span>
</code></pre>

<h1 id="DESCRIPTION">DESCRIPTION</h1>

<p>BIO_f_md() returns the message digest BIO method. This is a filter BIO that digests any data passed through it, it is a BIO wrapper for the digest routines EVP_DigestInit(), EVP_DigestUpdate() and EVP_DigestFinal().</p>

<p>Any data written or read through a digest BIO using BIO_read_ex() and BIO_write_ex() is digested.</p>

<p>BIO_gets(), if its <b>size</b> parameter is large enough finishes the digest calculation and returns the digest value. BIO_puts() is not supported.</p>

<p>BIO_reset() reinitialises a digest BIO.</p>

<p>BIO_set_md() sets the message digest of BIO <b>b</b> to <b>md</b>: this must be called to initialize a digest BIO before any data is passed through it. It is a BIO_ctrl() macro.</p>

<p>BIO_get_md() places the a pointer to the digest BIOs digest method in <b>mdp</b>, it is a BIO_ctrl() macro.</p>

<p>BIO_get_md_ctx() returns the digest BIOs context into <b>mdcp</b>.</p>

<h1 id="NOTES">NOTES</h1>

<p>The context returned by BIO_get_md_ctx() can be used in calls to EVP_DigestFinal() and also the signature routines EVP_SignFinal() and EVP_VerifyFinal().</p>

<p>The context returned by BIO_get_md_ctx() is an internal context structure. Changes made to this context will affect the digest BIO itself and the context pointer will become invalid when the digest BIO is freed.</p>

<p>After the digest has been retrieved from a digest BIO it must be reinitialized by calling BIO_reset(), or BIO_set_md() before any more data is passed through it.</p>

<p>If an application needs to call BIO_gets() or BIO_puts() through a chain containing digest BIOs then this can be done by prepending a buffering BIO.</p>

<p>Calling BIO_get_md_ctx() will return the context and initialize the BIO state. This allows applications to initialize the context externally if the standard calls such as BIO_set_md() are not sufficiently flexible.</p>

<h1 id="RETURN-VALUES">RETURN VALUES</h1>

<p>BIO_f_md() returns the digest BIO method.</p>

<p>BIO_set_md(), BIO_get_md() and BIO_md_ctx() return 1 for success and 0 for failure.</p>

<h1 id="EXAMPLES">EXAMPLES</h1>

<p>The following example creates a BIO chain containing an SHA1 and MD5 digest BIO and passes the string &quot;Hello World&quot; through it. Error checking has been omitted for clarity.</p>

<pre><code> <span class="variable">BIO</span> <span class="variable">*bio</span><span class="operator">,</span> <span class="variable">*mdtmp</span><span class="operator">;</span>
 <span class="variable">char</span> <span class="variable">message</span><span class="operator">[]</span> <span class="operator">=</span> <span class="string">"Hello World"</span><span class="operator">;</span>
 
 <span class="variable">bio</span> <span class="operator">=</span> <span class="variable">BIO_new</span><span class="operator">(</span><span class="variable">BIO_s_null</span><span class="operator">());</span>
 <span class="variable">mdtmp</span> <span class="operator">=</span> <span class="variable">BIO_new</span><span class="operator">(</span><span class="variable">BIO_f_md</span><span class="operator">());</span>
 <span class="variable">BIO_set_md</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="variable">EVP_sha1</span><span class="operator">());</span>
 <span class="regex">/*
  * For BIO_push() we want to append the sink BIO and keep a note of
  * the start of the chain.
  */</span>
 <span class="variable">bio</span> <span class="operator">=</span> <span class="variable">BIO_push</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="variable">bio</span><span class="operator">);</span>
 <span class="variable">mdtmp</span> <span class="operator">=</span> <span class="variable">BIO_new</span><span class="operator">(</span><span class="variable">BIO_f_md</span><span class="operator">());</span>
 <span class="variable">BIO_set_md</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="variable">EVP_md5</span><span class="operator">());</span>
 <span class="variable">bio</span> <span class="operator">=</span> <span class="variable">BIO_push</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="variable">bio</span><span class="operator">);</span>
 <span class="regex">/* Note: mdtmp can now be discarded */</span>
 <span class="variable">BIO_write</span><span class="operator">(</span><span class="variable">bio</span><span class="operator">,</span> <span class="variable">message</span><span class="operator">,</span> <span class="variable">strlen</span><span class="operator">(</span><span class="variable">message</span><span class="operator">));</span>
</code></pre>

<p>The next example digests data by reading through a chain instead:</p>

<pre><code> <span class="variable">BIO</span> <span class="variable">*bio</span><span class="operator">,</span> <span class="variable">*mdtmp</span><span class="operator">;</span>
 <span class="variable">char</span> <span class="variable">buf</span><span class="operator">[</span><span class="number">1024</span><span class="operator">]</span><span class="operator">;</span>
 <span class="keyword">int</span> <span class="variable">rdlen</span><span class="operator">;</span>
 
 <span class="variable">bio</span> <span class="operator">=</span> <span class="variable">BIO_new_file</span><span class="operator">(</span><span class="variable">file</span><span class="operator">,</span> <span class="string">"rb"</span><span class="operator">);</span>
 <span class="variable">mdtmp</span> <span class="operator">=</span> <span class="variable">BIO_new</span><span class="operator">(</span><span class="variable">BIO_f_md</span><span class="operator">());</span>
 <span class="variable">BIO_set_md</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="variable">EVP_sha1</span><span class="operator">());</span>
 <span class="variable">bio</span> <span class="operator">=</span> <span class="variable">BIO_push</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="variable">bio</span><span class="operator">);</span>
 <span class="variable">mdtmp</span> <span class="operator">=</span> <span class="variable">BIO_new</span><span class="operator">(</span><span class="variable">BIO_f_md</span><span class="operator">());</span>
 <span class="variable">BIO_set_md</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="variable">EVP_md5</span><span class="operator">());</span>
 <span class="variable">bio</span> <span class="operator">=</span> <span class="variable">BIO_push</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="variable">bio</span><span class="operator">);</span>
 <span class="keyword">do</span> <span class="operator">{</span>
     <span class="variable">rdlen</span> <span class="operator">=</span> <span class="variable">BIO_read</span><span class="operator">(</span><span class="variable">bio</span><span class="operator">,</span> <span class="variable">buf</span><span class="operator">,</span> <span class="variable">sizeof</span><span class="operator">(</span><span class="variable">buf</span><span class="operator">));</span>
     <span class="regex">/* Might want to do something with the data here */</span>
 <span class="operator">}</span> <span class="keyword">while</span> <span class="operator">(</span><span class="variable">rdlen</span> <span class="operator">&gt;</span> <span class="number">0</span><span class="operator">);</span>
</code></pre>

<p>This next example retrieves the message digests from a BIO chain and outputs them. This could be used with the examples above.</p>

<pre><code> <span class="variable">BIO</span> <span class="variable">*mdtmp</span><span class="operator">;</span>
 <span class="variable">unsigned</span> <span class="variable">char</span> <span class="variable">mdbuf</span><span class="operator">[</span><span class="variable">EVP_MAX_MD_SIZE</span><span class="operator">]</span><span class="operator">;</span>
 <span class="keyword">int</span> <span class="variable">mdlen</span><span class="operator">;</span>
 <span class="keyword">int</span> <span class="variable">i</span><span class="operator">;</span>
 
 <span class="variable">mdtmp</span> <span class="operator">=</span> <span class="variable">bio</span><span class="operator">;</span>   <span class="regex">/* Assume bio has previously been set up */</span>
 <span class="keyword">do</span> <span class="operator">{</span>
     <span class="variable">EVP_MD</span> <span class="variable">*md</span><span class="operator">;</span>
 
     <span class="variable">mdtmp</span> <span class="operator">=</span> <span class="variable">BIO_find_type</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="variable">BIO_TYPE_MD</span><span class="operator">);</span>
     <span class="keyword">if</span> <span class="operator">(!</span><span class="variable">mdtmp</span><span class="operator">)</span>
         <span class="keyword">break</span><span class="operator">;</span>
     <span class="variable">BIO_get_md</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="operator">&amp;</span><span class="variable">md</span><span class="operator">);</span>
     <span class="keyword">printf</span><span class="operator">(</span><span class="string">"%s digest"</span><span class="operator">,</span> <span class="variable">OBJ_nid2sn</span><span class="operator">(</span><span class="variable">EVP_MD_type</span><span class="operator">(</span><span class="variable">md</span><span class="operator">)));</span>
     <span class="variable">mdlen</span> <span class="operator">=</span> <span class="variable">BIO_gets</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">,</span> <span class="variable">mdbuf</span><span class="operator">,</span> <span class="variable">EVP_MAX_MD_SIZE</span><span class="operator">);</span>
     <span class="keyword">for</span> <span class="operator">(</span><span class="variable">i</span> <span class="operator">=</span> <span class="number">0</span><span class="operator">;</span> <span class="variable">i</span> <span class="operator">&lt;</span> <span class="variable">mdlen</span><span class="operator">;</span> <span class="variable">i</span><span class="operator">++)</span> <span class="keyword">printf</span><span class="operator">(</span><span class="string">":%02X"</span><span class="operator">,</span> <span class="variable">mdbuf</span><span class="operator">[</span><span class="variable">i</span><span class="operator">]</span><span class="operator">);</span>
     <span class="keyword">printf</span><span class="operator">(</span><span class="string">"\n"</span><span class="operator">);</span>
     <span class="variable">mdtmp</span> <span class="operator">=</span> <span class="variable">BIO_next</span><span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">);</span>
 <span class="operator">}</span> <span class="keyword">while</span> <span class="operator">(</span><span class="variable">mdtmp</span><span class="operator">);</span>
 
 <span class="variable">BIO_free_all</span><span class="operator">(</span><span class="variable">bio</span><span class="operator">);</span>
</code></pre>

<h1 id="BUGS">BUGS</h1>

<p>The lack of support for BIO_puts() and the non standard behaviour of BIO_gets() could be regarded as anomalous. It could be argued that BIO_gets() and BIO_puts() should be passed to the next BIO in the chain and digest the data passed through and that digests should be retrieved using a separate BIO_ctrl() call.</p>

<h1 id="HISTORY">HISTORY</h1>

<p>Before OpenSSL 1.0.0., the call to BIO_get_md_ctx() would only work if the BIO was initialized first.</p>

<h1 id="COPYRIGHT">COPYRIGHT</h1>

<p>Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.</p>

<p>Licensed under the OpenSSL license (the &quot;License&quot;). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <a href="https://www.openssl.org/source/license.html">https://www.openssl.org/source/license.html</a>.</p>


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